Organometallic Electron-Transfer Salts with Tetracyanoethylene

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with Tetracyanoethylene Exhibiting Ferromagnetic Coupling Joel S. Miller and Arthur J. Epstein 1

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Central Research and Development, Ε. I. du Pont de Nemours and Company, Experimental Station-E328, Wilmington, D E 19880-0328 Department of Physics and Department of Chemistry, The Ohio State University, Columbus, O H 43210-1106

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Some molecular organic solids comprising linear chains of alter­ nating total spin angular momentum quantum number S = / metallocenium donors, D, and cyanocarbon acceptors, A (i.e., • • • D · + A · - D · + A · - • • • ) exhibit cooperative magnetic phenom­ ena (i.e., ferro-, antiferro-, ferri-, and metamagnetism). For [FeIII(C Me ) ]• [TCNE]• (Me is methyl; TCNE is tetracyanoeth­ ylene), bulk ferromagnetic behavior is observed below the Curie tem­ perature of 4.8 K. Replacement of Fe with Cr , Ni , andFe leads to complexes with antiferromagnetic coupling, ferrimagnetic behav­ ior, and almost no magnetic interaction, respectively. These results are consistent with a model of configuration mixing of the lowest charge-transfer excited state with the ground state developed earlier to understand the magnetic coupling of such systems. The model, which predicts the magnetic coupling as a function of electron con­ figuration and direction of charge transfer, is a useful guide in the design of new organic and organometallic complexes with cooperative magnetic coupling. New TCNE-based electron-transfer salts were prepared to test the model and identify new materials with ferro­ magneticcoupling. 1

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L O L E C U L A R A N D ORGANIC F E R R O M A G N E T I C C O M P O U N D S , a l t h o u g h p o s ­

t u l a t e d i n the 1960s, have o n l y recently b e e n s y n t h e s i z e d a n d c h a r a c t e r i z e d 0065-2393/90/0226-0419$06.00/0 © 1990 American Chemical Society

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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E L E C T R O N TRANSFER IN BIOLOGY A N D T H E SOLID STATE

(1-5). T h i s d e v e l o p m e n t , w h i c h parallels t h e discovery o f m o l e c u l a r a n d organic-based superconductors, extends the study o f cooperative p h e n o m e n a i n m o l e c u l a r organic materials. T h e b r o a d range o f p h e n o m e n a i n t h e m o ­ lecular organic s o l i d state, c o m b i n e d w i t h t h e a n t i c i p a t e d modification o f p h y s i c a l properties v i a c o n v e n t i o n a l synthetic organic c h e m i s t r y a n d t h e ease o f fabrication enjoyed b y soluble materials, m a y u l t i m a t e l y l e a d to t h e i r use i n future generations o f e l e c t r o n i c a n d p h o t o n i c devices. T h i s chapter summarizes t h e configuration m i x i n g o f a v i r t u a l t r i p l e t

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excited state w i t h the g r o u n d state for an a l t e r n a t i n g d o n o r - a c c e p t o r

(D-A)

o n e - d i m e n s i o n a l c h a i n m o d e l for the stabilization o f ferromagnetic c o u p l i n g of this electron-transfer c o m p o u n d (1-6). A discussion o f the c o m m o n i d e a l ­ i z e d magnetic behaviors e x p e c t e d i n materials (2) a n d a m o r e c o m p r e h e n s i v e discussion o f several models for ferromagnetic c o u p l i n g i n m o l e c u l a r p o l ­ y m e r i c materials can b e f o u n d i n p u b l i s h e d reviews (1-5).

Stabilization of Ferromagnetic Coupling by Configuration Mixing S p i n a l i g n m e n t t h r o u g h o u t t h e s o l i d is necessary for b u l k f e r r o m a g n e t i s m . S e v e r a l m e c h a n i s m s ( J - 5 ) have b e e n p r o p o s e d for the p a i r w i s e stabilization o f ferromagnetic c o u p l i n g a m o n g spins. H o w e v e r , these schemes are insuf­ ficient

to account for t h r e e - d i m e n s i o n a l ferromagnetic behavior. A m e c h a ­

n i s m to account for this t h r e e - d i m e n s i o n a l interaction is d e s c r i b e d i n this chapter. T h e m o d e l o f configuration m i x i n g o f a v i r t u a l t r i p l e t charge-transfer excited

state

with

the ground

state

for a · · · 0 * Α * " 0 * Α * ~ Ο +

+

β

+

Α * · · · c h a i n to stabilize ferromagnetic c o u p l i n g was o r i g i n a l l y i n t r o d u c e d b y M c C o n n e l l (6)> F o r a D * A " ~ p a i r w i t h a half-occupied nondegenerate +

highest o c c u p i e d m o l e c u l a r o r b i t a l ( H O M O ) , t h e spins c o u p l e antiferromagnetically ( F i g u r e 1, la). ( P r e s u m a b l y , the v i r t u a l charge transfer involves o n l y t h e highest-energy partially o c c u p i e d m o l e c u l a r o r b i t a l ( P O M O ) . C i r ­ cumstances i n w h i c h v i r t u a l excitation from a l o w e r - l y i n g filled (or to a h i g h e r l y i n g filled) o r b i t a l dominate t h e a d m i x i n g e x c i t i n g state are conceivable, a n d t h e o r b i t a l degeneracy a n d s y m m e t r y restrictions are relaxed.) A d m i x t u r e o f the h i g h e r e n e r g y charge-transfer states w i t h t h e g r o u n d state lowers the total electronic energy a n d stabilizes antiferromagnetic c o u ­ p l i n g . F i g u r e 1 ( l i b a n d l i e ) illustrates ai* e l e c t r o n b e i n g d e l o c a l i z e d onto an adjacent site. T h i s energy r e d u c t i o n a n d d e r e a l i z a t i o n does n o t occur w h e n the two e l e c t r o n spins are p a r a l l e l (ferromagnetically aligned), i n accord w i t h t h e P a u l i exclusion p r i n c i p l e . T h u s , antiferromagnetic a n d (for o t h e r electron configurations) ferromagnetic c o u p l i n g can b e a c h i e v e d along a n d b e t w e e n chains (2, 5, 7, 8).

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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M I L L E R & EPSTEIN

A"

D

(la)-· ·

+

4

(lb).. ·

D

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Organometallic Electron-Transfer Salts

+

A"

A"

+

421

4

-

4

4

·•

(Ic).. ·

4

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(Da)...

4

44

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(Hb).. ·

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ΊΤ1Τ

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«•.••44- 44- 44.14 44 -44- 44- 44 #4 44

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Figure 1. Schematic illustration of stabilization of antiferromagnetic or fer­ romagnetic coupling. If both the D and A have a half-filled nondegenerate POMO (s ) (la), then the A*~D (or D«-A) charge-transfer excited state (lb or equivalently Ic) stabilizes antiferromagnetic coupling. If either D or A has a non-half-filled degenerate POMO (e.g., d , assumed here to be the D) (Ha), then the D w i t h tetracyanoethylene, 2 ( M e is m e t h y l ; T C N E n

5

5

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CH

3

is tetracyanoethylene) (9, 10). T h i s electron-transfer salt possesses b o t h the alternating · · · D * A * ~ D A "~D * A *~ · · · ( F i g u r e 2) crystal a n d e l e c t r o n i c structures p r e s c r i b e d b y the configuration m i x i n g m e c h a n i s m already d e ­ s c r i b e d (9). +

β +

+

T h e h i g h - t e m p e r a t u r e susceptibility of [ F e ( C M e ) r [ T C N E ] - fits the C u r i e - W e i s s expression w i t h θ = + 3 0 Κ ( F i g u r e 3) a n d indicates d o m ­ inant ferromagnetic interactions (9). T h e s u s c e p t i b i l i t y a n d saturation m a g ­ netization calculated as the s u m of the contributions from [ F e ( C H ) 2 ] * p a r a l l e l to the C m o l e c u l a r axis a n d [ T C N E ] is 6.46 m i l l i e l e c t r o m a g n e t i c u n i t s p e r m o l e ( m e m u / m o l ) at 2 9 0 Κ a n d 1 6 . 7 e l e c t r o m a g n e t i c u n i t s * k i l o g r a m p e r m o l e ( e m u k G / m o l ) , respectively. T h e s e values are i n excellent agreement w i t h the o b s e r v e d values of 6.67 m e m u / m o l a n d 16.3 e m u k G / m o l for single crystals a l i g n e d p a r a l l e l to the c h a i n axis (10). m

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+

m

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A spontaneous magnetization is o b s e r v e d for p o l y c r y s t a l l i n e samples b e l o w 4.8 Κ i n the E a r t h ' s magnetic field (9). T h e magnetization of these crystals is 3 6 % greater than that of i r o n m e t a l o n a p e r - i r o n basis, a n d it agrees w i t h the calculated saturation m o m e n t for ferromagnetic a l i g n m e n t of the d o n o r a n d the acceptor spins. T h e c r i t i c a l (Curie) t e m p e r a t u r e , T , is 4.8 K , a n d hysteresis loops characteristic of ferromagnetic materials are o b s e r v e d (1-5). A large coercive f i e l d of 1 k G is r e c o r d e d at 2 Κ (10). T h e p h y s i c a l properties are s u m m a r i z e d i n T a b l e I. Single-crystal susceptibility can be c o m p a r e d w i t h different p h y s i c a l c

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

M I L L E R & EPSTEIN

Organometallic Electron-Transfer Salts

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22.

Figure 2. Alternating donor-acceptor, · · · D ' A · · · ), linear chain structure of [Fe (C Me ) ]' [A]'(A is TCNQ, TCNE, DDQ, or C (CN) ), [Fe (C5H ) ][TCNE], and [Fe (C Me ) ]' [C (CN) ]'-. The structure shows adjacent out-of-registry chains for TCNE. +

m

lI

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m

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m o d e l s to a i d the u n d e r s t a n d i n g of microscopic s p i n interactions. F o r s a m ­ ples o r i e n t e d p a r a l l e l to the field, the susceptibility above 16 Κ fits a o n e d i m e n s i o n a l H e i s e n b e r g m o d e l w i t h a ferromagnetic exchange, / , o f 19 c m " (JO). V a r i a t i o n o f the l o w - f i e l d magnetic susceptibility w i t h t e m p e r a t u r e for an u n u s u a l l y b r o a d temperature range above T [χ oc (Τ - T ) ~ ] , m a g n e t i 1

c

c

7

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

424

E L E C T R O N TRANSFER IN BIOLOGY A N D T H E SOLID STATE

900

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800 -

Ε ô Ε

700 •

ICo »(C5Me5)2l tTCNE] l

+

θ

β

·1 Κ

y

*

600

CL

δ

CO 3 C0

500

400

Ο Φ C

f

[ΝΙ«>(05Μθ )2]·ηΤ0ΝΕ]· 5

θ s -10 κ

300

α ο

e

200

DC

100

ο. ο φ

[Fe (C5Me5)2]-+[TCNEK m

θ s +30 Κ

300

Temperature, Τ, Κ Figure 3. Reciprocal susceptibility, x~ , extrapolated from the high-tempera­ ture data for [M (C Me ) ]' [TCNE]-[M is Fe (ferromagnetic: θ = 30 Κ), Ni (antiferromagnetic: θ = - 1 0 Κ), and Co (paramagnetic: θ = -1 Κ)]. J

UI

5

5

2

+

zation w i t h t e m p e r a t u r e b e l o w T [ M °c ( T - T ) " ] , a n d t h e magnetization w i t h magnetic field at T (Μ « Η ) e n a b l e d t h e estimation o f the β , 7, a n d δ c r i t i c a l exponents. T h e values o f 1.2, ~ 0 . 5 , a n d 4.4, r e s p e c t i v e l y , w e r e d e t e r m i n e d for t h e magnetic field p a r a l l e l to t h e c h a i n axis. T h e s e values are consistent w i t h a m e a n - f i e l d - l i k e t h r e e - d i m e n s i o n a l b e h a v i o r . T h u s , above 16 K , o n e - d i m e n s i o n a l nearest-neighbor s p i n interactions are sufficient c

c

c

p

1 / δ

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

57

Additional magnetic behaviors observed:

+

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[TCNE]'"

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C H N Fe l - D - · - D ' A - - D - A - - D Α · · · · Chains Conventional organic solvents 4.8 Κ + 30 Κ Yes, in zero applied field 0.00667 emu/mol (obs, 290 K) 0.00640 emu/mol (calc, 290 K) 0.00180 emu/mol (obs, 290 K) 0.00177 emu/mol (calc, 290 K) 16,300 emuG/mol (calc 16,700 emuG/mol); 36% greater than iron (iron basis) 6,000 emuG/mol (calc 8,800 emuG/mol) 27.4 Κ (19 cm' ) 8.1 Κ (5.6 c m ) Yes (1000 G coercive field; cf. 1 G for iron metal) 0.5 (cf. 0.38 for iron metal) 1.22, 1.19 (cf. 1.33 for iron metal) 4.4 (cf. unknown for iron metal) Yes; neutron diffraction studies on polycrystalline deuterated samples Yes; in zero applied field [large internal field: 424,000 G (4.2K)] New mechanism for ferromagnetism appears to be operative; predictive model based upon configurational mixing of the lowest charge transfer excited state developed Chemical modification leads to meta-, antiferro-, ferri-, para-, and diamagnetic behavior

5

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MIL

Formula Structure Solubility Critical/ Curie temperature Curie-Weiss θ constant Spontaneous magnetization Magnetic susceptibility (|| to l - D chains) Magnetic susceptibility (|| to l - D chains) Magnetic susceptibility ( 1 to l - D chains) Magnetic susceptibility ( 1 to l - D chains) Saturation magnetization (|| to l - D chains) Saturation magnetization (X to l - D chains) Intrachain exchange interaction (|| to l - D chains) Intrachain exchange interaction (X to l - D chains) Hysteresis curves β Critical constant y Critical constant (|| and X to l - D chains) δ Critical constant Ferromagnetic ordering Fe Mossbauer Zeeman splitting Physical model

5

Table I. Physical Properties of [ F e ( C M e ) J ·

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22.

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

426

E L E C T R O N T R A N S F E R IN B I O L O G Y A N D T H E SOLID STATE

to u n d e r s t a n d t h e magnetic c o u p l i n g , b u t near T

C

three-dimensional spin

interactions are d o m i n a n t (10). The

5 7

F e M o s s b a u e r spectra o f t h e T C N E

Fe(C Me ) 5

5

2

electron-transfer salt o f

are informative. A t y p i c a l six-line Z e e m a n split spectra are o b ­

s e r v e d i n zero a p p l i e d magnetic field at l o w t e m p e r a t u r e as the radical anions p r o v i d e a n i n t e r n a l d i p o l a r field. F o r e x a m p l e , a Z e e m a n split s p e c t r u m w i t h a n i n t e r n a l field o f 424 k G (4.2 K ) is o b s e r v e d for t h e [ T C N E ] " " (9) salt. T h e i n t e r n a l fields are substantially greater than the expectation o f 110

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kG/spin/Fe. S i m i l a r l y s t r u c t u r e d electron-transfer complexes based o n T C N E a n d organometallic donors w e r e investigated i n an effort to u n d e r s t a n d the s t r u c ­ t u r a l features necessary to stabilize t h e i r b u l k ferromagnetic b e h a v i o r . O u r study i n c l u d e d complexes w i t h s u b s t i t u t i o n o f the M e groups o n t h e c y c l o pentadienide ring w i t h H , increasing the ring size to six b y u s i n g bis(arene)chromium, a n d substitution o f F e w i t h R u a n d O s . Stable radicals are n e e d e d to f o r m ferromagnetically c o u p l e d chains. T h u s , e l e c t r o n transfer m u s t occur to enable c l o s e d - s h e l l donors a n d accep­ tors to b e candidates for magnetic materials. T h e o n e - e l e c t r o n s o l u t i o n r e ­ v e r s i b l e r e d u c t i o n p o t e n t i a l , E°, provides a means to gauge w h e t h e r o r n o t e l e c t r o n transfer m i g h t occur for a s o l i d . F o r e x a m p l e , ferrocene is m o r e difficult to oxidize (by 0.5 V ) than decamethylferrocene, a n d i t is u n a b l e to r e d u c e T C N E (11-15). N e v e r t h e l e s s , t h e diamagnetic ferrocene analog o f [ F e ( C M e ) r [ T C N E ] * ~ (i.e., [ F e " ( C M e ) ] [ T C N E ] ) forms (12-14) a n d m

5

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2

+

5

5

2

possesses t h e i d e n t i c a l structural m o t i f (16-18) ( F i g u r e 3). E i t h e r a t e m ­ p e r a t u r e - o r p r e s s u r e - i n d u c e d " n e u t r a l - i o n i c " transition (19-22) m i g h t b e sufficient to l e a d to t h e stabilization o f ferromagnetic behavior. H o w e v e r , above 2 Κ at a m b i e n t p r e s s u r e , o n l y F e " is o b s e r v e d v i a M o s s b a u e r spec­ troscopy, a n d n o d i s c o n t i n u i t y is o b s e r v e d i n t h e s u s c e p t i b i l i t y data (15). The C o "

1

analog, [ C o

(C Me ) ]

i n

5

5

2

, +

[ T C N E ] - , has b e e n p r e p a r e d a n d

exhibits essentially the C u r i e s u s c e p t i b i l i t y anticipated for S = V2 [ T C N E ] (Θ = - 1 . 0 K ) (9). Because t h e cation is diamagnetic, t h e electron-transfer c o m p l e x has o n l y o n e s p i n p e r f o r m u l a u n i t . It appears that the · · · ϋ * Α * ~ +

D " A * " » · · structure type w i t h b o t h S ^ V2O a n d S ^ V2 A is necessary, +

b u t insufficient, for stabilizing cooperative h i g h l y magnetic behavior. A t ­ tempts to p r e p a r e [ M ( C M e ) r m

5

5

2

+

( M is R u , Os) salts o f [ T C N E ] ' " have

yet to l e a d to suitable c o m p o u n d s for c o m p a r i s o n w i t h the h i g h l y magnetic Fe

phase (23). F o r m a t i o n o f [ R u ( C M e 5 ) ] '

m

n i

p o r t i o n a t i o n to R u " ( C M e ) 5

Os

1 1 1

5

2

5

and [ R u

2

I V

is c o m p l i c a t e d b y d i s p r o ­

+

(C Me )(C Me CH )] 5

5

5

4

2

+

(24). T h e

analog l e d to t h e p r e p a r a t i o n o f a salt w i t h T C N E ; h o w e v e r , l o w sus­

c e p t i b i l i t i e s a n d crystals unsuitable for single-crystal X - r a y studies (23) have

hampered

progress

i n this

[ F e ( C M e ) r [ T C N E ] - with N i m

5

5

2

+

area. m

Replacement

(S = Vfc) o r C r

m

of

Fe

1 1 1

in

(S = %) leads to

c o m p o u n d s e x h i b i t i n g cooperative magnetic properties (25). T h e m o t i v a t i o n for s t u d y i n g these complexes emanated from the m o d e l

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

22.

M I L L E R & EPSTEIN

Organometallic Electron-Transfer

427

Salts

for the stabilization o f ferromagnetic c o u p l i n g i n m o l e c u l a r solids (26). A n ­ tiferromagnetic c o u p l i n g is p r e d i c t e d for d - s J

complexes w i t h s

s y m m e t r y , S >: 1/2 radidals w i t h a degenerate P O M O are r e q u i r e d . It is a challenge to the synthetic c h e m i s t to p r e p a r e radicals that have nondegenerate P O M O s and do not u n d e r g o a J a h n - T e l l e r d i s t o r t i o n , w h i c h w o u l d e l i m i n a t e the d e s i r e d electronic configuration. I n a d d i t i o n to p r e p a r a t i o n of the d e s i r e d radicals, t h e i r secondary a n d tertiary solid-state structures m u s t b e a c h i e v e d . F i n a l l y , single crystals large e n o u g h for the study of t h e i r anisotropic m a g ­ netic properties m u s t b e p r e p a r e d . 2d

3

Acknowledgments T h e authors gratefully acknowledge partial support b y the D e p a r t m e n t of E n e r g y D i v i s i o n of M a t e r i a l s Science ( G r a n t N o . D E - F G 0 2 - 8 6 E R

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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E L E C T R O N TRANSFER IN BIOLOGY A N D T H E SOLID STATE

45271.A000). W e d e e p l y thank o u r co-workers (R. W . B i g e l o w , J . C . C a l abrese, S. C h i t t i p e d d i , A . C h a k r a b o r t y , K . M i n g - C h i , K . R . C r o m a c k , D . A . D i x o n , P. J . K r u s i c , V . L . G o e d k e n , D . M . O ' H a r e , W . M . Reiff, H . R o m m e l m a n n , C . V a z q u e z , M . D . W a r d , D . W i p f , a n d J . H . Zhang) for t h e i m p o r t a n t c o n t r i b u t i o n s t h e y have m a d e t o w a r d t h e success o f t h e w o r k reported herein.

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RECEIVED for review May 1, 1989. A C C E P T E D revised manuscript October 10, 1989.

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.